in this video, we're going to begin our lesson on measuring growth by plate counts. And so recall from our previous lesson videos that viable cells are really just living cells that are capable of multiplying on growth media. Now counting viable cells or living cells can have an advantage over direct cell counting because recall that direct cell counting includes both viable cells and non viable or dead cells as well. And so it may be the case that the scientist is only interested in the viable cells. And so in those cases scientists need to use other methods other than direct cell counting. And so plate counts will require the use of solid growth media in a Petri dish or in a plate and uh the plate or the Petri dishes where the cells are grown and counted on. And once again these cells that are counted on what these plate counts are going to represent viable or living cells. And so in our next lesson video will be able to talk more about the method and process for plate counts. But for now, this year concludes our brief introduction to measuring growth by plate counts, and we'll be able to get some practice and learn more as we move forward, so I'll see you all in our next video.

In this video, we're going to talk more about the process of plate counts. And so plate counts really just refers to the process of counting the number of viable or living cells in a culture. After those cells have been plated onto a Petri dish with solid growth media. Now, it is important to emphasize that once again, plate counts will only allow for the determination of the number of viable cells or living cells, which is, or can be an advantage over direct cell counting, which tends to include non viable or dead cells as well. And so the process of plate counts does have a really critical assumption. And so it does assume that each individual colony of cells that forms on the Petri dish or plate is going to be formed from a single individual cell. And so once again, this is a really, really important assumption In order for uh, this plate count process to work now in this process of plate counts, the cells are originally grown in a liquid culture or in a liquid broth. And then those cells ultimately will be transferred to a solid growth media on a Petri dish or on a plate. And then that Petri dish or plate will be incubated to ultimately determine the number of colony forming units, or c F. U. S for short. And so a colony forming unit or a C. F. You really just represents a single viable cell that is capable of multiplying to form a colony of cells. And so ultimately, the number of cf US, or colony forming units is going to represent the number of viable cells that were initially added to the plate. And so ultimately what we're saying here is by counting the number of colonies, you can determine the number of colony forming units and the number of colony forming units. Through this important assumption is going to represent the number of viable cells. And so a scientist can determine or count the number of viable or living cells through this plate count process. Now it is important that the liquid culture that the cells are originally grown in must be diluted appropriately using serial dilution ins, which is really just a set of back to back delusions. And this is all in order to get an easy or reasonable number of colony forming units to count. And so what this means is that the number of colony forming units cannot be too high because then it will be really hard to count, but it also cannot be too low because then it may not be reliable enough to count. And so there has to be an appropriate number of colony forming units. Usually that number is somewhere between 30 and 300 colony forming units as you'll see down below in our image. And so it is really important for the scientists to know which dilution in the serial set of serial dilutions that they actually use to count the number of colony forming units, because this will allow them to accurately calculate the number of colony forming units and the number of viable cells. And so if we take a look at this image down below, we can get a better understanding of the process of plate count preparation and so notice that towards the top here, what we have are a bunch of test tubes and these test tubes really contain liquid media, liquid culture media, so they're containing the cells growing in a liquid broth. And so over here on the far left, this is the original tube where the cells were originally grown in. And so usually if you take this original liquid culture broth containing these cells and if you transfer the cells from the liquid broth to a Petri dish to grow them. Usually there's going to be way too many colony forming units to accurately count them. And so you can see each of these little circles that you see here represents a colony forming unit and there's simply just way too many to count here. You're going to make a mistake if you try to count the number of colony forming units in the original liquid culture and so usually what's required is the original liquid culture needs to go through. Dilution is in fact serial dilutions, a set of back to back delusions and that's what we're seeing here with the rest of these tubes are a bunch of dilution. So usually what you do is a 1 to 10 dilution at first where you take one millimeter of the original liquid culture media With the cells and you dilute it into nine million liters of liquid bra to do a 1-10 dilution. And so usually when you play the 1 to 10 dilution, you may also get way too many colonies to accurately count. And so that's why the serial dilutions are continuously done in order for you to get a reasonable number of colony forming units to count. So you can do another 1 to 10 dilution Of this 1-10 dilution, which which ultimately gives you a 1-100 dilution of the original liquid culture. Um And so when you play these, you may actually get a reasonable or a accountable plate With a countable number of colony forming units somewhere between 30 and 300 colonies. And so the scientists could count the number of colonies. And again the number of colonies represents the number of colony forming units. And the number of colony forming units represents the number of viable cells. And so once they count the number of viable, uh uh once they have the number of viable cells in this particular dilution and they know how much of that dilution they played it, they can reverse calculate how many viable cells are in this original liquid culture. And so what you can see here in this last tube is just another serial dilution, which gives you a 1 to 1000 dilution of the original liquid culture. But notice that when plating this 1 to 1000 dilution that there are not enough cells, there's only three here. And so that's not going to be uh something that is reliable. So you really want To make sure that you have colony forming units between 30 and 300 on the plate. And so really this process of plate counts allows for the determination of the number of viable cells or the number of living cells. And so this year concludes our brief lesson on plate counts, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.